The present invention relates to a system and a method for controlling the NOX levels in flue gases emitted from boilers combusting carbonaceous fuels. More particularly, the present invention relates to an NOX control scheme that is free from injecting an external NOX reducing agent.
NOX emissions from carbonaceous fuel-firing boilers originate from two sources: (1) thermal NOX due to oxidation of nitrogen in the air and (2) fuel NOX due to oxidation of nitrogen in the fuel. In today's boilers, with advanced combustion systems, thermal NOX is minimal, and NOX emissions are mainly formed from a small fraction of nitrogen in the fuel. The level of NOX produced in a combustion process is mainly determined by the temperature and stoichiometry of the primary combustion zone. The level of NOX emissions exiting from a combustor to the atmosphere results as an equilibrium between the NOX formation reactions and NOX reduction reactions.
Existing technologies for controlling NOX emissions from combustion sources fall within two categories: (1) minimizing the NOX formation in the combustion process and (2) reducing the the NOX level in the produced flue gas. In pulverized coal (PC) boilers, the NOX formation can be minimized by using specially designed low NOX burners (LNB) and by completing the coal combustion at the upper level of the furnace by over-fire-air (OFA). In fluidized bed combustion (FBC), the NOX levels are usually controlled by using a relatively low combustion temperature and by adjusting secondary air for optimized air staging. The main flue gas NOX reduction technologies include selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR), which both usually utilize ammonia or urea to destroy NOX, once it has formed.
Today's new coal-fired utility boilers typically have NOX emissions in the range of 40–60 ppm. These low levels of NOX emissions are achieved by the optimized integration of both categories of NOX control technologies. For example, a common arrangement for PC boilers is an LNB/OFA system in combination with an SCR using ammonia or urea as the reductant. When using an LNB/OFA system, the NOX level at the exit of the furnace is typically in the range of 90–180 ppm.
The current low NOX technologies used in carbonaceous fuel-combusting boilers emphasize the precise control of combustion stoichiometry and temperature within the primary combustion zone. It is well known that a low level of excess air in the combustion zone may lead to increased CO emissions and unburned carbon in the ash. Thus, the current low NOX combustion technologies (LNB and FBC) are, due to CO emission concerns, unable to take full advantage of optimizing the amount of excess air. The currently used design strategy has thus been focused on reducing the available oxygen in the primary combustion zone to a low level to minimize NOX formation while at the same time maintaining high combustion efficiency and a low level of CO emissions.
A problem with the SCR and SNCR reduction systems, however, is that the use of excessive amounts of ammonia or urea to achieve very high NOX reduction levels leads to harmful ammonia emissions to the environment. Ammonia handling and injection systems create significant capital and operational costs. The use of ammonia also causes safety risks to the operating personnel, and may result in ammonia salt formation, and fouling and corrosion on cold downstream surfaces of the flue gas channel.
In the automotive industry, it is known to use the so-called Three-way Converters (TWC) to simultaneously reduce the NOX, CO and hydrocarbon (HC) emissions in the exhaust gas. The conventional gasoline engine runs at stoichiometric conditions, controlled by fuel injection. A TWC contains a catalyst, which is usually made of either platinum or palladium together with rhodium on a ceramic or metal substrate. CO functions as the NOX reductant over the rhodium surface. The excess CO and hydrocarbons are oxidized over the platinum or palladium surfaces.
U.S. Pat. No. 5,055,278 discloses a method of decreasing the amount of nitrogen oxides in waste furnace gas. According to the method, fossilized fuel is passed through gradual pyrolizing combustion for prolonged residence time, and the formed carbon monoxide, hydrocarbons, and possible nitrogen oxides, are passed through catalytic oxidation. Due to the substoichiometric conditions, very high amounts of CO and hydrocarbons are produced, and a large amount of catalyst is required for the oxidation. Contrary to that, the amount of nitrogen oxides in the waste gas is very low, because fuel nitrogen is mainly released as NH3. For this reason, air or oxygen has to be injected as an external oxidant, and mixed uniformly with the flue gas, upstream of the catalyst. The pyrolizing combustion method also suffers from low thermal efficiency due to high levels of unburned carbon in the ash.
For the above-mentioned reasons, there is clearly a need for a new, simple system level integration between the combustion process of a boiler and the downstream flue gas NOX reduction, which maintains high thermal efficiency and leads to very low NOX emissions, but does not cause harmful ammonia or CO emissions.